Chiral hydrogenating catalysts

ABSTRACT

A stereo-differentiating catalyst containing Group VIII metal is prepared by contacting a Group VIII metal-containing acidic zeolite with an optically active amine. Methods are disclosed for utilizing the catalyst in hydrocarbon conversion processes, e.g., hydrogenation and hydroformylation.

Stereoisomers are chemical substances having the same molecular formulawhile differing in their arrangement of atoms in space. Enantiomers arestereoisomers which are mirror images of each other and which have thesame physical and chemical properties except for their optical activity,i.e., an ability to rotate a plane of polarized light. Optically activedextrorotatory isomers (+ or d) rotate a plane of polarized lightclockwise while levorotatory isomers (- or l) rotate the plane of lightcounterclockwise.

Addition of more than one asymmetric center into a molecule producesdiastereomers that do not have the same chemical or physical properties.Diastereomers are stereoisomers that are not mirror images of eachother.

Because the biological activities of many compounds are influenced bystereochemical factors it is often desirable to produce such opticallyactive materials. However, special precautions must be taken to ensureproduction of an optically active product because of the tendency toproduce optically inactive racemic mixtures, that is equal amounts ofeach mirror image stereoisomer whose opposite optical activities cancelout each other. For example, when an olefin, which in its saturated formis optically active, is hydrogenated, the usual resultant product isoptically inactive due to formation of the racemic mixture. In order toobtain the desired enantiomer or mirror image stereoisomer, the mixturemust be separated into its optically active components. This separation,known as optical resolution may be carried out by actual physicalsorting or by direct crystallization of racemic mixtures. However, themost common form of optical resolution involves formation ofdiastereomeric derivatives by means of an optically active resolvingagent. Because the resulting diastereomers, unlike enantiomers havedifferent physical properties, they may be separated by various methodsincluding fractional crystallization, gas-liquid chromatography, thinlayer chromatography and liquid chromatography. However, such opticalresolution procedures are often laborious, expensive as well asdestructive to the undesired enantiomorph. Due to these difficulties,increased attention has been placed upon asymmetric synthesis in whichone of the enantiomorphs is obtained in significantly greater amounts.In asymmetric synthesis a chiral unit in an ensemble of substratemolecules is converted by a reactant into a chiral unit in such a mannerthat the stereoisomeric products are produced in unequal amounts. Aprochiral function serves as the precursor for a chiral product duringthe reaction. Further information relating to asymmetric synthesis maybe found in Kirk-Othmer, Encyclopedia of Chemical Technology, ThirdEdition, Volume 17, pages 311-345, Wiley-Interscience, New York, N.Y.,1982.

Catalytic asymmetric hydrogenation catalysts are known in the art. Forexample, U.S. Pat. No. 4,265,827, discloses a process for thehomogeneous catalytic hydrogenation of certain organic acids to producean optically active mixture. Hydrogenation is carried out in thepresence of an optically active coordinated metal complex hydrogenationcatalyst in which the metal is selected from the group consisting ofrhodium, iridium, ruthenium, osmium, palladium, and platinum. Suchcatalysts are soluble in the reaction mixture and are known as"homogeneous" catalysts.

U.S. Pat. No. 4,273,933 discloses a modified nickel catalyst employedfor stereo-diffentiating reduction of carbonyl compounds. The modifiednickel catalyst is prepared by soaking a nickel catalyst in an aqueousmodifying medium having dissolved therein an inorganic salt, and anoptically active substance such as optically active hydroxy acid. Thenickel catalyst may be supported on a conventional carrier, such asdiatomaceous earth or alumina.

The present invention relates to a process for preparingasymmetric-hydrogenation catalysts which comprises contacting a GroupVIII metal-containing acidic zeolite with an optically active amine.Suitable zeolites include those shape-selective zeolites of intermediatepore size having Constraint Index values from about 1 to 12 as well aslarge pore size zeolites having a Constraint Index value less than about1.

The catalysts prepared in accordance with the present invention areparticularly suitable for preferentially converting one optical isomerrelative to its enantiomer as well as the preferential formation of anoptically active product from a non-chiral feed.

Asymmetric hydrogenation catalysts of the present invention may beprepared by incorporating a Group VIII metal into a zeolitic material,and if necessary, exposing the resulting metal-containing zeolite toreducing conditions so as to limit the susceptibility of theincorporated Group VIII metal to migration and agglomeration. Wheremetal has been incorporated into the zeolite in a partially reducedstate, e.g., Rh⁺ in the form of rhodium carbonyl chloride, a reductionstep is unnecessary. The Group VIII metal-containing zeolitic materialis then reacted with a chiral or optically active amine in order toneutralize some of the acidic sites present.

Zeolites can be described as crystalline metallosilicates consisting ofa rigid three dimensional framework of SiO₄ or an MO₄ where M is a GroupIII A metal, for example, aluminum or gallium, wherein the tetrahedraare cross-linked by the sharing of oxygen atoms such that the ratio ofthe total Group III A metal and silicon atoms to oxygen is 1:2. Theelectrovalence of the tetrahedra containing Group III A metal isbalanced by the inclusion in the crystal of a cation, for example, analkali metal or an alkaline earth metal cation. This can be expressedwherein the ratio of metal M to the number of various cations, such asCa/2, Sr/2, Na, K or Li is equal to unity. One type of cation may beexchanged either entirely or partially by another type of cationutilizing ion exchange techniques in a conventional manner. By means ofsuch cation exchange, it has been possible to vary the properties of agiven zeolite by suitable selection of the cation.

Prior art techniques have resulted in the formation of a great varietyof synthetic zeolites. These zeolites have come to be designated byletter or other convenient symbols, as illustrated by zeolite X (U.S.Pat. No. 2,882,244) zeolite Y (U.S. Pat. No. 3,130,007), ZK-5 (U.S. Pat.No. 3,247,195,) ZK-4 (U.S. Pat. No. 3,314,752), zeolite beta (U.S. Pat.No. 3,308,069), ZSM-5/ZSM-11 intermediate compositions (U.S. Pat. No.4,229,424), ZSM-5 (U.S. Pat. No. 3,702,886), ZSM-11 (U.S. Pat. No.3,709,979), ZSM-12 (U.S. Pat. No. 3,832,449) ZSM-23 (U.S. Pat. No.4,076,842), ZSM-35 (U.S. Pat. No. 4,016,245), ZMS-38 (U.S. Pat. No.4,046,859) and ZSM-48 (U.S. Pat. No. 4,375,573), merely to name a few.All of the above patents are incorporated herein by reference.

Another suitable zeolite, ZSM-50 has a formula, on an anhydrous basisand in terms of moles of oxides per 100 moles of silica, as follows:

    (0-4)R.sub.2 O:(0-10)M.sub.2/n O:(1-5)Al.sub.2 O.sub.3 :(100)SiO.sub.2

wherein M is an alkali or alkaline earth metal, n is the valence of M,and R is an organic cation of a Group VA element of the Periodic Tableof the Elements (Sargent-Welch Scientific Company), particularly thatderived from a linear diquaternary ammonium, phosphonium, arsonium,stibonium or bismuthonium compound having the general formula:

    [X(CH.sub.3).sub.3 M'(CH.sub.2).sub.6 M'(CH.sub.3).sub.3 X]

wherein X is a halide anion (e.g. fluoride, chloride, bromide oriodide).

Catalytically active members of the family of the ZSM-50 crystals have adefinite X-ray diffraction pattern which distinguishes them from othercrystalline materials. The X-ray diffraction pattern of zeolite ZSM-50has the following significant lines:

                  TABLE 1                                                         ______________________________________                                        Interplanar                                                                   d-spacing (A)                                                                              Relative Intensity, I/Io                                         ______________________________________                                        20.1 ± .3 W                                                                11.1 ± .17                                                                              S                                                                10.1 ± .16                                                                              M                                                                 9.7 ± .14                                                                              W                                                                5.77 ± .09                                                                              W                                                                5.61 ± .09                                                                              W                                                                4.64 ± .07                                                                              M                                                                4.35 ± .07                                                                              M                                                                4.30 ± .07                                                                              VS                                                               4.00 ± .06                                                                              S                                                                3.85 ± .06                                                                              M                                                                3.70 ± .06                                                                              M                                                                3.42 ± .05                                                                              W                                                                3.35 ± .05                                                                              W                                                                3.27 ± .05                                                                              M                                                                3.24 ± .05                                                                              W                                                                2.94 ± .04                                                                              W                                                                2.53 ± .04                                                                              W                                                                ______________________________________                                    

In Table I, the relative intensities are given in terms of the symbolsW=weak, M=medium, S=strong and VS=very strong. In terms of intensities,these may be generally designated as follows:

W=0-20

M=20-40

S=40-60

VS=60-100

ZSM-50 can be prepared from a reaction mixture containing sources of analkali or alkaline earth metal oxide, an oxide of aluminum, an oxide ofsilicon, an organic cation of a Group VA element of the Periodic Tableand water and having a composition, in terms of mole ratios of oxides,falling within the following ranges:

    ______________________________________                                        Reactants       Useful   Preferred                                            ______________________________________                                        SiO.sub.2 /Al.sub.2 O.sub.3                                                                    20-100  30-90                                                OH.sup.- /SiO.sub.2                                                                            0.1-0.6 0.1-0.3                                              R/SiO.sub.2     0.05-0.6 0.1-0.3                                              M/SiO.sub.2     0.01-1.0 0.1-0.6                                              ______________________________________                                    

wherein R and M are as above defined.

The ZSM materials and zeolite beta noted above are shape-selectivematerials having a high silica content. These aluminosilicates have asilica to alumina mole ratio of at least 12, say about 70 or even about100 or 200 or greater. Such materials also have a Constraint Indexwithin the range of about 1-12. Methods for determination of ConstraintIndex are well-known and are set out in the Journal of Catalysis 67,218-222 (1981).

The zeolites of the present invention contain a Group VIII metal as ahydrogenating agent. The Group VIII metal is added to the zeolite byconventional impregnation methods, which may include ion exchange. GroupVIII metals may be incorporated within the zeolite by ion exchange bydissolving a Group VIII metal or metal compound in a carrier liquidwhich is not only a good solvent for the desired metal complex but whichalso does not destroy the zeolite or where present, the zeolite binder.Preferred solvents are of molecular dimensions such that they can freelypenetrate the pores of the zeolite selected. Between about 5 and 100% byweight, preferably between about 10 and 90% by weight of the originalmetal cations in the zeolite are exchanged with the solubilized or metalcompound. Impregnation may be carried out by preparing an aqueous ororganic solvent solution of the metal complex and contacting the zeolitepreferably in the acidic form, with the solution at a temperaturebetween the freezing point of the solvent and about 100° C., such thatthe resulting zeolite contains from about 0.1 to 10% by weight of metal,preferably about 0.1 to 1% by weight. Depending on its intended use, thezeolitic material upon removal from the contacting solution may bewashed with a suitable solvent.

Representative ion exchange techniques are disclosed in a wide varietyof patents, including U.S. Pat. No. 3,142,249; U.S. Pat. No. 3,142,251;and U.S. Pat. No. 3,140,253. These patents are incorporated herein byreference.

If necessary, after the metal incorporation step, the zeolite may besubjected to reducing conditions in order to reduce at least some of themetal incorporated within the zeolite in such a way as to limit metalmigration and agglomeration. Metal reduction can be accomplished bycontacting the metal impregnated zeolite with a suitable reducing agentsuch as hydrogen at temperatures ranging from about 20° to about 500°C., preferably about 300° to about 500°, and pressures ranging fromabout 1 to about 20 atm., preferably about 1 to about 10 atmospheres. Inthose cases where the zeolite contains a form of platinum or palladium,it is preferred that the zeolite be calcined by gradually increasing thezeolite from ambient temperature to about 350° C. in the presence ofoxygen. The reducing agent may be combined with an inert diluentmaterial such as nitrogen. The reduction process results in a Group VIIImetal containing zeolite containing acidic sites resulting fromreduction of the metal cation, e.g., in the case of platinum tetramineloaded zeolites:

    H.sub.2 +Pt(NH.sub.3).sub.4 →Pt°+2H.sup.+ +4NH.sub.3 ↑.

After reduction, the Group VIII metal-loaded zeolite is contacted withan optically active chiral amine. Contacting conditions includetemperatures ranging from about 100°-600° C., preferably about 300°-500°C. The optically active chiral amines suitable for use in the presentinvention are characterized by an ability to enter the zeolite pores.Examples of such amines include S-(-)-alpha-methylbenzylamine or(S)-(-)-2-methylbutylamine, (S)-(+)-alanine (S)-(+)-valine, and (R)-(-)valine. The zeolite materials are contacted with the optically activeamine for a sufficient time to permit neutralization of some of theacidic sites of the zeolite. The amount of amine added should be suchthat it occupies about 20-50% of the pore volume of the zeolite. Theresulting optically active material may be further treated by combiningit with a suitable inorganic matrix such as silica, alumina orsilica-alumina.

The resulting product of the present invention is suitable for use as astereo-differentiating hydrogenation catalyst. Such asymmetrichydrogenation may be used in the preferential conversion of one opticalisomer relative to its enantiomer, in the preferential formation of anoptically active product from a non-chiral feed, such as an opticallyactive alcohol from a ketone, or in the hydroformylation of an olefinicmaterial.

The invention can be further illustrated by the following examples whichare to be understood as exemplifying specific embodiments of the presentinvention without limiting the same.

EXAMPLE 1 Preparation of Group VIII Metal Containing Zeolite treatedwith S-(-)-α-Methylbenzylamine

As synthesized ZSM-5 is ion-exchanged with ammonium chloride to produceNH₄ ZSM-5, NH₄ ZSM-5 is then ion-exchanged with platinum by contact witha Pt⁺ (NH₃)₄ (NO₃)₂ solution overnight. The resulting material iselevated from room temperature to 350° C. in the presence of oxygen at arate of about 0.5° C./minute. The calcined material is then reduced inthe presence of hydrogen gas to form a zeolite which contains acidicsites. 10 g of the reduced acidic platinum-loaded product is contactedwith 0.3 g of optically active S-(-)-alpha-methylbenzylamine dissolvedin toluene to form a platinum loaded ZSM-5 zeolite having acid sitesneutralized by the amine.

EXAMPLE 2 Conversion of 2-Phenylbutene To Optically Active2-Phenylbutane

2-Phenylbutene is hydrogenated in the presence of the product of Example1 and hydrogen under hydrogenation conditions. The reduction product,2-phenylbutane exhibits optical activity.

EXAMPLE 3 Conversion of Acetylphenone To Optically Active1-Phenylethanol

Acetophenone is contacted with hydrogen gas in the presence of thehydrogenation catalyst prepared in accordance with Example 1. Reductionis effected under conventional reduction conditions. The product ofreduction is optically active 1-phenylethanol.

EXAMPLE 4 Preparation of Rhodium Hydroformylation Catalyst ContainingOptically Active Amine

As synthesized ZSM-5 is ion exchanged with ammonium chloride to produceNH₄ ZSM-5, which is then treated with a solution of rhodium carbonylchloride dimer in mesitylene.

10 g of the rhodium-loaded product is contacted with 0.3 g of opticallyactive S-(-)-alpha-methylbenzylamine dissolved in toluene to form arhodium loaded ZSM-5 zeolite having acid sites neutralized by the amine.

EXAMPLE 5 Hydroformylation of α-Methylstyrene

An autoclave is charged with 10 g of the catalyst of Example 4 and 50 gof α-methylstyrene. The contents of the autoclave are then heated to100° C. and a 50/50 mixture of carbon monoxide and hydrogen is added togive a pressure of 1000 psig, which is maintained by periodic additionof the carbon monoxide-hydrogen mixture. After about 2 hours, thereaction mixture is withdrawn and found to contain optically activealdehydes.

What is claimed is:
 1. A process for preparing an asymmetrichydrogenation catalyst which comprises contacting a Group VIIImetal-containing acidic zeolite with an optically active amine.
 2. Theprocess of claim 1 wherein said zeolite has a silica to alumina ratio ofless than about 300 and a Constraint Index of less than about
 12. 3. Theprocess of claim 2 wherein said zeolite has a silica to alumina ratio ofat least about 12 and a Constraint Index ranging from about 1 to
 12. 4.The process of claim 1 wherein said Group VIII metal is selected fromthe group consisting of Pt, Pd, Ru and Rh.
 5. The process of claim 1wherein said optically active amine is selected from the groupconsisting of (S)-(-)-alpha-methylbenzylamine and(S)-(-)-2-methylbutylamine.
 6. The process of claim 1 wherein saidoptically active amine is selected from the group consisting of(S)-(+)-alanine, (S)-(+)-valine and (R)-(-)-valine.
 7. The process ofclaim 1 wherein said zeolite is selected from the group consisting ofzeolite X, zeolite Y and ZSM-20.
 8. The process of claim 1 wherein saidzeolite is selected from the group consisting of zeolite beta, ZSM-5,ZSM-5/ZSM-11 intermediate, ZSM-11, ZSM-12, ZSM-23, ZSM-35, ZSM-38,ZSM-48 and ZSM-50.
 9. A process for preparing asymmetric hydrogenationcatalysts which comprises incorporating a Group VIII metal into azeolite, and reacting said material with an optically active amine. 10.The process of claim 9 wherein the metal-containing zeolite is exposedto reducing conditions prior to said reacting with an optically activeamine.
 11. A chiral hydrogenating catalyst which is prepared bycontacting a Group VIII-metal containing acidic zeolite with anoptically active amine.